1. Field of the Invention
The present invention relates to a heat exchanger used for a refrigeration system, an air conditioner, and so forth, and to a method and an apparatus for fabricating the heat exchanger.
2. Description of the Prior Art
FIG. 64 is a perspective view showing a conventional heat exchanger used for air conditioning disclosed in Japanese Patent Publication (kokai) No. 61-153388, and FIG. 65 is a sectional view of FIG. 64. In FIGS. 64 and 65, reference numeral 1 means heat transfer tubes, and 2 is small-gage wires connected to the heat transfer tubes 1 so as to serve as a fin. Reference mark A means out-tube operating fluid (such as air), and B is in-tube operating fluid (such as coolant). In the heat exchanger, the small-gage wires 2 thread through the heat transfer tubes 1 disposed in parallel, and mutually intersect. That is, the heat exchanger has a mesh-type structure including the heat transfer tubes 1 serving as the warp, and the small-gage wires 2 serving as the weft.
A description will now be given of the operation. As shown in FIG. 65, in the heat exchanger used for air conditioning in the conventional embodiment, when the out-tube fluid A flows between the small-gage wires 2 threading through the heat transfer tubes 1, the small-gage wires 2 disturb a flow of the out-tube fluid A. That is, as shown by the arrow of FIG. 65, the out-tube fluid A flowing directly below the small-gage wire 2 collides with the small-gage wire 2, and is divided into right and left flows. Besides, there is another flow of the out-tube fluid A upward moving on a surface of the heat transfer tube 1 along the small-gage wire 2. This results in a long contact time between the out-tube operating fluid A and the heat transfer tubes 1, that is, a long contact time between the out-tube operating fluid A and the in-tube operating fluid B.
In order to fabricate such a heat exchanger, the small-gage wires 2 may thread through the plurality of heat transfer tubes 1 disposed in parallel with the small-gage wires 2 mutually intersecting. That is, the heat exchanger is assembled to have the mesh-type structure including the heat transfer tubes 1 serving as the warp, and the small-gage wires 2 serving as the weft. Further, after the assembly of the mesh-type structure, contact portions between the heat transfer tubes 1 and the small-gage wires 2 are welded one by one in order to enhance thermal conductivity.
FIG. 66 shows another conventional embodiment, i.e., a plate fin-type heat exchanger used for a room air conditioner and so forth. For assembly of the heat exchanger, instead of the small-gage wires 2 serving as a fin, plate-type fins are mounted at interval of about 1 to 5 mm. Further, a heat transfer tube 1 is inserted into a hole provided in the fin, and after the insertion, fluid is introduced into the heat transfer tube 1 with pressure. Thereby, a diameter of the heat transfer tube 1 is expanded to bring the heat transfer tube 1 into tight contact with the plate fin 102.
In the plate fin-type heat exchanger, the out-tube operating fluid A can flow along the plate fin without large turbulence, resulting in reduced thermal conductivity.
In recent years, a diameter of the heat transfer tube has been decreased in order to provide a more compact and higher-performance heat exchanger. However, when the narrow heat transfer tube is applied to the heat exchanger (in particular, an evaporator), a higher pressure loss is caused in a coolant flowing in the tube, resulting in a reduced performance of an air conditioner. Hence, in a typical method, the number of path of the heat exchanger is increased to decrease an amount of circulating coolant per path, thereby avoiding the reduction of performance.
Typically, a branch pipe may be used for several paths. For several tens to several hundreds paths, in many cases, an inlet header and an outlet header are mounted, and a plurality of heat transfer tubes are disposed between the headers so as to provide a multi-path heat exchanger (evaporator).
FIG. 67 is a sectional view of a conventional multi-path evaporator disclosed in Japanese Patent Publication (Kokai) No. 6-26737. In the drawing, reference numeral 3a means an inlet header, and 3b is an outlet header. Reference numeral 45 means inlet coolant piping. The inlet coolant piping is a straight pipe whose length is equal to or less than twenty times a bore diameter of an expansion valve 58, and an irregular surface 61 is provided in the inlet coolant piping 45.
FIGS. 68 and 69 are font views of a conventional gas-liquid separating heat exchanger disclosed in Japanese Patent Publication (Kokai) No. 6-117728. In FIG. 68, an opening in a lower inlet header 3a is coupled with an opening in an upper outlet header 3b through a gas-liquid separating cylinder 63 having a predetermined length. Further, reference numeral 2 means a plurality of mesh fins mounted to heat transfer tubes 1 in a direction perpendicular to a heating surface, and 64 is a throttle valve mounted to avoid a counter-flow in the vicinity of a connecting portion of the outlet header 3b to the gas-liquid cylinder 63. A gas-liquid two-phase coolant flows through inlet coolant piping 45, and is vertically divided into two phases, i.e., an upper gaseous phase and a lower liquid phase, by a difference in gravity therebetween in the gas-liquid separating cylinder 63. The gas coolant is bypassed through the outlet header 3b, and only the liquid coolant is supplied into the heat transfer tubes 1 through the inlet header 3a. Thus, the liquid can uniformly be distributed to paths.
As shown in FIG. 69, a float pipe 66 vertically passes through the gas-liquid separating cylinder 63 to form a liquid level controller 65. Further, a cylindrical float 67 is fitted into the float pipe 66 so as to vertically move according to a variation in liquid level of the liquid coolant in a gas-liquid separating chamber. A plurality of openings 68 and 69 are provided in both upper and lower ends of the float pipe 66. In such a structure, the gas coolant can be bypassed through the openings 68 in the upper end to the outlet header 3b.
FIG. 70 is a sectional view of a conventional multi-path evaporator disclosed in Japanese Patent Publication (Kokai) No. 6-159983. In the drawing, a plurality of coolant dispersing holes 71 are provided in a peripheral wall of a coolant dispersing tube body 70, and the coolant dispersing tube body 70 is disposed in an inlet header 3a. A liquid coolant introduced into the inlet header 3a through the coolant dispersing holes 71 can be distributed to the heat transfer tubes 1.
FIG. 71 is a front view of a conventional multi-path evaporator disclosed in Japanese Patent Publication (Kokai) No. 6-101935. In the drawing, a plurality of heat transfer tubes 1 are vertically disposed in parallel to each other, and an inlet header 3a and an outlet header 3b are connected through the heat transfer tubes 1. Further, an upper portion of the inlet header 3a and an upper portion of the outlet header 3b are communicated through a gas bypass pipe 72.
The conventional heat exchanger used for air conditioning has the above structure. In the heat exchanger used for air conditioning, the heat transfer tube itself has a narrow width in the range of 1 to 5 mm. The heat exchanger has greater heat transfer coefficient than that of a heat exchanger used in a conventional room air conditioner. However, for the same front surface area, the heat exchanger has too small heating surface area which is equal to or less than one fifth of a heating surface area of the heat exchanger used in the conventional room air conditioner. Consequently, there is a problem in that a required amount of heat exchange can not be obtained. In order to overcome the problem, it can be considered to use a plurality of rows of heating surfaces. However, when the conventional heat exchangers as described above are used in a plurality of rows, an air side pressure loss becomes high, and an air flow is reduced in spite of the same fan power. Thus, there is the problem in that the required amount of heat exchange can not be obtained. In particular, there is another problem in that the above tendency becomes significant when the heat exchanger is used as an evaporator, and vapor in air condenses on the heating surface.
Further, non-azeotropic mixed coolant can be used in the heat transfer tube. In this case, the plurality of rows of heat exchangers may be arranged, and the non-azeotropic mixed coolant may sequentially be supplied to the heat exchangers starting from the back row. The non-azeotropic mixed coolant may be a cross flow serving as a spurious counter-flow with respect to a flow of the air. It is known that this technique can provide a considerably enhanced performance. However, an increase in the number of row increases a width of the heat exchanger, resulting in a larger unit. Hence, at the most, only two rows of the heat exchangers can be used in the conventional room air conditioner and so forth. Thus, there are problems in that, for example, it is extremely difficult to provide the cross flow serving as the spurious counter-flow.
Further, in the conventional heat exchanger, after the completion of assembly of the heat transfer tubes and the small-gage wires in the mesh-type structure, tight contact becomes insufficient between the heat transfer tube and the small-gage wire. Consequently, thermal conductivity between the heat transfer tube and the small-gage wire is reduced. As a result, there is a problem of reduction of heat exchanging ability.
Further, after the heat transfer tubes and the small-gage wires are assembled in the mesh-type structure, the heat transfer tubes the small-gage wires are welded for assembly. When the assembly is completed, the heat exchanging ability can be enhanced. However, since each connecting portion between the heat transfer tube and the small-gage wire should discretely be welded, fabrication requires vast amount of labor. As a result, there is another problem in that it is difficult to realize mass production.
Further, in order to connect the heat transfer tube to the header, each connecting portion therebetween should discretely be welded so that fabrication requires vast amount of labor. In addition, since supply of welding material can not be controlled, an excess or a shortage is caused in welding material, resulting in insufficient tight contact between the heat transfer tube and the header. As a result, there is a further problem in that the heat exchanging ability is reduced due to leakage of the in-tube operating fluid.
Further, the heat transfer tubes and the small-gage wires are assembled in the mesh-type structure so that the small-gage wires mutually intersect between the heat transfer tubes. Hence, water generated by dehumidification can not drop from an intersecting portion, and a flow of the out-tube operating fluid A is disturbed. As a result, there is a problem of reduction of the heat exchanging ability.
Further, in the plate fin-type heat exchanger, the out-tube operating fluid A can flow along the plate fin without large turbulence, resulting in reduced heat transfer coefficient. As a result, there is a problem in that the heating surface area of the heat exchanger, that is, the heat exchanger itself must be made larger so as to compensate for the reduced heat transfer coefficient.
Further, in the conventional multi-path heat exchanger, when the coolant flows into the inlet header through the inlet coolant piping, in many cases, the coolant forms a wave-like flow in which a gaseous phase and a liquid phase are separated from one another in the inlet header. Hence, when the coolant flows into the plurality of heat transfer tubes, an inhomogeneous distribution of a coolant flow is caused. In addition, there are some heat transfer tubes into which only the gaseous phase flows, and at which heat exchange can not effectively be performed. As a result, there is a still further problem in that an area used for effective heat exchange (hereinafter referred to as effective heating surface area) becomes smaller than an actual heating surface area.
Further, in the conventional heat exchanger, the coolant is expanded by the expansion valve to form a homogeneous two-phase flow, and passes through the coolant piping including the straight pipe to flow into the inlet header. Therefore, the coolant forms the homogeneous two-phase flow at an inlet portion of the header. However, in many cases, the coolant is decelerated at a time of inflow, the gaseous phase and the liquid phase are gradually separated from one another, and the coolant finally forms the wave-like flow. Hence, at a portion other than the inlet portion of the inlet header, there is caused the inhomogeneous distribution of the coolant flowing into the plurality of heat transfer tubes. In addition, since the gaseous phase and the liquid phase are separated from one another, only the gaseous phase flows into some of the heat transfer tubes. As a result, there is a still further problem of reduction of the effective heating surface area.
Further, the conventional heat exchanger requires special means such as gas-liquid separating cylinder. As a result, there are problems in that the heat exchanger has a complicated structure, and the coolant can not smoothly flow because the coolant flows into the headers after division into the gaseous phase and the liquid phase.
Further, in the conventional heat exchanger, it is necessary to provide an additional dispersing tube body in the header. Further, a flow velocity is more decreased toward the inner side of the inlet header in its longitudinal direction. As a result, there is a further problem in that the coolant can not uniformly be distributed.
Further, in the conventional heat exchanger, inlet coolant piping 5 is mounted at a lower position of the inlet header, and outlet coolant piping 6 is mounted at an upper portion of the outlet header 2. Thus, the heat exchanger requires a large mounting space in a unit, and is inconveniently mounted to an air conditioner which is long from side to side. In addition, since the coolant flows in a direction from a lower portion to an upper portion in the inlet header, the coolant can not sufficiently be distributed to the upper portion of the header in case of a reduced flow rate. As a result, there is a problem in that a control is required to sufficiently distribute the coolant to the heat transfer tubes.